In a mixing bowl, combine flour, sugar and baking powder. Add egg, milk, butter and lemon peel; mix just until dry ingredients are moistened. Fold in the blueberries. Spread in a greased 9-in. square baking pan.
For topping, combine sugar, flour, walnuts and cinnamon. Cut in butter until mixture is crumbly. Sprinkle over batter.
Bake at 350 degrees F (176 degrees Celsius) for 40-45 minutes or until cake tests done.
Frost is a common problem with wild blueberries and is the primary cause of a poor harvest. Not all blueberry farms have the same climate. The fields in Quebec and Maine blossom before the fields on Prince Edward Island and are therefore more susceptible to frost. The later blossoming of Island berries usually occurs after the last frost of the spring.